It is not clear to me whether you have spongy brakes or a low brake pedal. Spongy brakes come from compressable gasses in the hydraulic circuit. This gas is called air, and gets in when the circuit has been opened. You didn't say if someone has been working on the brakes. Also I have to assume you are talking about a car with 4 wheel discs. Loose wheel bearings can cause low pedal. A failing master cylinder is characterised by a pedal that slowly sinks toward the floor under pressure.
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